The reaction of trans-4-hydroxy-2-nonenal, a major alpha, beta-unsaturated aldehyde released during lipid peroxidation, with deoxyguanosine under physiological conditions was investigated in order to assess its DNA damaging potential. This aldehyde was dissolved in tetrahydrofuran (THF) prior to addition to the reaction mixture. The results showed that structurally different adducts were formed in these reactions depending on the THF used. Using THF unprotected from light, reactions yielded adducts 1 to 6. Adduct 1 was characterized as 1,N2-ethenodeoxyguanosine (5,9-dihydro-9-oxo-3-beta-D-deoxyribofuranosylimidazo[1,2-alpha]pu rine) by its UV, proton nuclear magnetic resonance, and mass spectrum and by comparison to the corresponding guanosine and guanine adducts reported in the literature. The UV spectrum of adduct 4 was indicative of a substituted 1,N2-etheno derivative. Adducts 2,3,5, and 6 were essentially identical in UV spectra and appeared to be N2-substituted deoxyguanosine diastereomers. At room temperature adducts 2,3,5, and 6 were converted quantitatively to a single product at pH 10.5. This product was shown to be identical to 1,N2-ethenodeoxyguanosine (adduct 1). Analogous conversions to 1,N2-ethenoguanine were also observed for the corresponding guanine adducts. Using THF that had been protected from the light, however, the reactions of trans-4-hydroxy-2-nonenal with deoxyguanosine gave three major adducts, 7,8, and 9. These adducts possessed UV spectra similar to that of 1,N2-propanodeoxyguanosine and were not converted to 1,N2-ethenodeoxyguanosine upon base treatment. Evidence obtained suggests that adducts 1 to 6 were formed from the reaction of deoxyguanosine with the epoxide of trans-4-hydroxy-2-nonenal generated in the presence of hydroperoxide in the light unprotected THF, whereas adducts 7 to 9 were formed by direct Michael addition. Adducts 1 to 6 were formed presumably as a result of nucleophilic addition of the exo-amino of deoxyguanosine to the aldehydic group of the epoxide of trans-4-hydroxy-2-nonenal. Base treatment of these adducts facilitated subsequent cyclization and eliminations and finally gave 1,N2-ethenodeoxyguanosine. These results demonstrated that trans-4-hydroxy-2-nonenal readily forms adducts with deoxyguanosine either by direct Michael addition or via its epoxide formation. The facile conversion of some of these adducts to a single adduct suggests that 1,N2-ethenodeoxyguanosine may provide a simple and useful marker for assessing potential DNA damage by trans-4-hydroxy-2-nonenal and related alkenals associated with lipid peroxidation.